In situ analysis of myelin basic protein gene expression in myelin-deficient oligodendrocytes: antisense hnRNA and readthrough transcription.

Mice homozygous for the recessive mutation myelin deficient (mld) exhibit a severe deficit in the synthesis of myelin basic protein (MBP). The primary defect in mld mice is an unusual rearrangement of the MBP gene. The mld MBP locus consists of two tandem MBP genes that span approximately 90 kb of DNA; the upstream gene contains an extensive inversion of its 3' region, while the downstream gene appears identical to the wild-type gene. In this report, the aberrant expression of the mld MBP locus was examined by in situ hybridization with MBP genomic and cDNA probes. In situ hybridization with a single-stranded genomic probe from the inverted region of the mld MBP gene revealed that primary transcripts initiating from the upstream MBP promoter elongate through the inverted region generating abundant antisense MBP heterogeneous nucleus RNA (hnRNA) transcripts in mld oligodendrocytes. Little or no antisense MBP RNA was detected in cytoplasmic regions in mld brain sections indicating that the antisense MBP hnRNA transcripts are not processed and transported out of the nucleus. Furthermore, we provide evidence that these abnormal transcripts elongate through the downstream MBP gene's promoter region, suggesting that transcription of the downstream mld MBP gene is inhibited by readthrough transcripts that originate at the upstream promoter.